The present invention relates to an idling control device for an internal combustion engine with a turbocharger; particularly it is useful when provided on an automobile engine.
As shown in FIG. 1, in a conventional idling control device for internal combustion engine with a turbocharger, one end of a bypass passage 20 for idling control is opened upstream of a turbocharger 3, and the other end of the passage 20 is opened to a surge tank 8 provided between the internal combustion engine 10 and the turbocharger 3. In this bypass passage 20 there is provided a flow control valve 12 which is actuated by an actuator 4 to be controlled by a computer 5. The flow control valve 12 controls the amount of air to be supplied into the engine at idling by changing the passage area of the bypass passage 20.
However, in the device of this construction, the downstream side of the flow control valve 12 (on the side of the surge tank 8) is exposed to vacuum and the upstream side thereof is exposed to the atmosphere, at idling operation. On the other hand, at the time of turbocharging, the downstream side of the valve is exposed to the turbopressure, and the upstream side thereof is exposed to the atmosphere. Therefore, the flow control valve 12 receives a rightward force in FIG. 1 because of the pressure difference at idling, and receives a leftward force in FIG. 1 at the time of turbocharging. When the flow control valve 12 receives a leftward force, the actuator 4 which actuates the control valve 12 has to complete with an excessive force. Moreover, the valve 12 provides a poor antivibration performance when turbocharging, because of the fact that the leftward force applied when in turbocharging operates to weaken the spring force of the spring 13 which is provided on the flow control valve 12 to urge it onto its valve seat. Further, when in turbocharging, the turbocharged air is applied directly to the flow control valve 12, therefore sometimes the turbocharged air leaks through a gap at the flow control valve 12 to the side of the atmosphere from the surge tank 8, thereby weakening the effect of turbocharging. When the turbocharged air leaks toward the atmosphere, the oil mist and soot in the surge tank 8 deteriorate the function of the flow control valve 12, since they tend to enter into the actuator 4.